app.js

angular.module('onlineModule',[]).factory('dataService', { ... });
angular.module('offlineModule',[]).factory('dataService', { ... });

var app = angular.module('myApp', ['onlineModule']); // onlineModule or offlineModule

app.service('x', function (dataService) {  // $dataService could be onlineModule.dataService or offlineModule.dataService
 // online or offline? we don't care!
});

app.service('y', function (dataService) {
// online or offline? we don't care!
});